1. Field of the Invention
The present invention relates to a printing method and a printing apparatus, and more particularly to a printing position adjustment for each of the printing elements arranged in a print head, which is performed in generating the print data.
2. Description of the Related Art
As an information output apparatus for a personal computer, a word processor or a facsimile, a printing apparatus such as a printer for printing information such as character or image on a printing medium such as paper or film-like sheet is widely employed. Though various printing methods for the printing apparatus are well known, an ink jet method has been widely employed in recent years because of the capability of non-contact printing on the printing medium such as paper, easy colorization, and low noise. In the ink jet printing apparatus, a serial printing method is generally employed in which a print head for ejecting ink is mounted on a carriage and the printing is performed while reciprocating the print head for a scan of the print head in a direction (hereinafter referred to as a main scanning direction) intersecting with a conveying direction of the printing medium.
In the serial method, a so-called bidirectional printing is performed in which the printing is performed by ejecting the ink from the print head in both forward and backward paths in moving the carriage on the printing medium, whereby the printing rate can be improved. Also, the printing rate can be improved by increasing the number of ink ejection openings (also called nozzles) arranged in the print head.
Meanwhile, there is conventionally a well-known problem of a misalignment of print positions which is caused by positions of ejection openings for respective kinds or colors of ink in a print head being displaced from each other. Particularly, this problem is more remarkable as the number of ejection openings arranged is increased as described above.
FIGS. 1A to 1C are views explaining displacements of nozzle between a plurality of print heads. These figures show the examples of respective print heads for ejecting respective color inks of K (black), C (cyan), M (magenta) and Y (yellow). FIG. 1A shows an example of a head arrangement in which the print heads for respective color inks have the same length (nozzle array length), and the respective print heads are arranged in the main scanning direction. Also, FIG. 1B shows an example of a head arrangement in which a part of the print heads for respective color inks has a different length, and the print heads are arranged in the main scanning direction. Further, FIG. 1C shows a head arrangement in which a part of the respective print heads for respective color inks has similarly a different length, and the print heads are arranged in the direction orthogonal to the main scanning direction.
Herein, if the positional relationship of corresponding nozzles between the color print heads is correct, inks ejected from the respective nozzles can form a correct positional relation on the printing medium to contribute to printing of a desired image. However, the correct positional relationship of nozzles may not be realized depending on the degree of precision of the print head in a manufacturing process or the degree of precision of mounting the print head on the printing apparatus. For example, in the head arrangement as shown in FIG. 1A, the C print head is displaced one nozzle downward, the M print head is displaced two nozzles upward, and the Y print head is displaced one nozzle upward, relative to the K head.
FIG. 2 is a view showing one example of the printed result using all the nozzles of the print head as shown in FIG. 1A. As shown in FIG. 2, a print area A101 is printed with M ink only, a print area A102 is printed with M and Y ink, a print area A103 is printed with K, M and Y inks, a print area A104 is printed with K, C and Y inks, a print area A105 is printed with K and C inks, and a print area A106 is printed with C ink only.
To reduce degraded image quality due to printing misalignment in a conveying direction (sub-scanning direction) of the printing medium, an adjustment (hereinafter called a vertical registration adjustment) of adjusting the printing position of the print head for each color is conventionally performed. The conventional vertical registration adjustments are well known in which nozzles used for printing are limited, a preset nozzle used for the vertical registration adjustment is employed, and print data is shifted according to an amount of misalignment in the printing position (e.g., refer to Japanese Patent Laid Open No. 5-104739 (1993) and Japanese Patent Laid Open No. 6-031909 (1994)).
However, in conventional printing apparatus that performs the vertical registration adjustment, the adjustment is performed by one of two methods: a method of limiting nozzles used for printing to perform the adjustment and a method of shifting print data to perform the adjustment.
In the case of limiting nozzles used for printing to perform the adjustment, since the nozzles used for printing are limited, a problem that a throughput by a printing apparatus is decreased may occur. On the other hand, in the vertical registration adjustment method of shifting the print data, when print data resolution is lower than nozzle resolution (a nozzle pitch), the vertical registration adjustment can be performed only at resolution lower than the nozzle resolution and therefore the deterioration of a printed image quality may occur.
Consequently, in the printing apparatus in which the vertical registration adjustment is performed in a condition that a range of nozzles used for printing is limited, printing is always performed with use of a limited number of nozzles and therefore a desired throughput may not be achieved. On the other hand, in the printing apparatus employing the vertical registration adjustment of shifting print data, when the print data resolution is lower than the nozzle resolution, the vertical registration adjustment can not be properly performed and therefore an image of desired print quality may not be obtained, regardless of how printing of high quality printed image is desired.